1. Field of the Invention
This invention relates to a marine propulsion unit for a watercraft, and more particularly to a hydraulic tilt and trim adjustment system for a marine propulsion unit.
2. Description of Related Art
The optimal trim angle of an outboard drive varies with a watercraft's running condition. For instance, the bow of the watercraft should press against the water when accelerating from rest or from a slow speed. To achieve this condition, the angle of the propeller shaft is disposed at a negative angle relative to the horizontal (i.e., at a negative trim angle). A thrust vector produced by the propeller in this position is thus out of the water. When running at high speed, the propeller is raised or trimmed to position the propeller shaft at a positive trim angle relative to the horizontal within the range of about 0.degree. to 15.degree.. The outboard drive also must be raised beyond the normal trim range in order to operate in shallow water and for storage in a full tilt-up position
A hydraulic tilt and trim adjustment system often adjusts the trim and tilt position of the outboard drive. The tilt and trim adjustment system usually includes at least one hydraulic actuator which essentially operates between the watercraft transom and the outboard drive unit. The actuator causes the outboard drive to pivot about a horizontal axis to raise or lower the outboard drive.
Tilt and trim adjustment systems also usually employ a hydraulic motor that effects the trim and tilt operations of the outboard drive. For this purpose, prior hydraulic motors have included a reversible electric motor that selectively drives a reversible fluid pump. The pump pressurizes or depressurizes the hydraulic actuator for raising or lowering the outboard drive.
In particular, the fluid pump supplies pressurized fluid to various ports of the actuator's closed cylinder, on either side of a piston which slides within the cylinder. The piston forms separate chambers within the cylinder. A conventional seal, such as one or more O-rings, operates between the piston and cylinder bore to prevent flow from between the chambers. The piston moves within the cylinder by pressurizing the chamber on one side of the piston and depressuring the other chamber on the opposite side.
An actuator arm is attached to the piston and to the outboard drive. The other end of the cylinder is attached to a bracket on the watercraft. By pressurizing and depressurizing the chambers within the actuator, the piston and thus the outboard drive can be moved.
The pressures in the cylinder chambers vary greatly depending on whether the propulsion unit is operating in a trim range or in a tilt range. In a tilt range, usually associated with tilting the propulsion unit out of the water, the pump generates a relatively low pressure in the chambers because the only load on the cylinder is the weight of the propulsion unit.
The pump conversely must generate far greater pressure to trim-up the motor because of the load placed on the unit by the propulsion unit. The increase in load results from the thrust of the propulsion unit. That is, a portion of the thrust produced by the propulsion unit acts downward and against the tilt and trim mechanism when trimming up. Higher pressures therefore are required in the cylinder to trim up the motor when running at high speeds (e.g., planning speeds). When used with pleasure boats (e.g., ski boats, sport boats, run-abouts, and the like), the tilt and trim adjustment systems are designed to trim the outboard drive relatively slowly to prevent drive "pop-up."
Undesirable motor pop-up occurs because the thrust of the propulsion system suddenly decreases as the motor is swung through the tilt range. Within the tilt range, the large pressure built-up within the cylinder rapidly pushes the piston upward and causes the outboard motor to pop-up quickly. Tilt and trim mechanisms used on pleasure type boats thus regulate trim and tilt-up speed.
For commercial applications, however, it often is desirable to quickly raise the outboard drive in order to avoid underwater articles, such as, for example, fishing nets and the like. The hydraulic circuitry employed with tilt and trim mechanisms used in commercial applications therefore permits the outboard drive to be raised quickly.
Because of the differences in the design of the hydraulic circuitry, it previously has not been easy to convert a tilt and trim adjustment system for commercial applications. For instance, U.S. Pat. No. 3,842,789 discloses a valve system which permits quick tilt and trim movement of an outboard drive unit; however, this system requires the manual control of a remote operator in order to actuate the valve and quickly raise the outboard drive.